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  <title>P0528R0: The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
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 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
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 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
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 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
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 * Miscellaneous
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 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

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/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

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/** Copyright *****************************************************************/

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/** Back to Top / ToC Toggle **************************************************/

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		/* See also Overflow section at the bottom */

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/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

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		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
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	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

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/** Section divider ***********************************************************/

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/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

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	 display: inline-block;
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	 line-height: 0.5em;
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/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
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	a > i { /* Instance of term */
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	dt dfn code, code.idl {
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	dfn var {
		font-style: normal;
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/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
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/** Miscellaneous improvements to inline formatting ***************************/

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		vertical-align: super;
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/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

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	pre {
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		margin-bottom: 1em;
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/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
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	/* Use distinguishing colors when user is interacting with the link */
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	/* Backout above styling for W3C logo */
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		text-decoration: none;
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/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
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	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

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/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

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/** Blockquotes ***************************************************************/

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/** Open issue ****************************************************************/

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	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
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/** Non-normative Note ********************************************************/

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	.note::before, .note > .marker,
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		color: hsl(120, 70%, 30%);
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	/* Add .note::before { content: "Note"; } for autogen label,
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		display: block;
		color: hsl(120, 70%, 30%);
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		border-bottom: 1px silver solid;
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/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
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		background: #EEE;
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/** Advisement Box ************************************************************/
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		display: block;
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	.advisement > .marker {
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/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
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	summary {
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	.annoying-warning:not(details),
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		z-index: 1000;
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}

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/** Entity Definition Boxes ***************************************************/

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		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
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/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
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/** Property/Descriptor Definition Tables *************************************/

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	/* A footnote at the bottom of a def table */
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		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
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/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

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		overflow-wrap: normal;
		hyphens: manual;
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		border-collapse: collapse;
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		margin: 0 auto 1em;
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	table.data  thead,
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P0528R0<br>The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="1970-01-01">1970-01-01</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0528r0">http://wg21.link/P0528r0</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:bigcheesegs@gmail.com">Michael Spencer</a> (<span class="p-org org">Sony Playstation</span>)
     <dt>Audience:
     <dd>SG1, LEWG, LWG, CWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P0528r0.bs">github.com/jfbastien/papers/blob/master/source/P0528r0.bs</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>I was thinking how nothing lasts—especially a struct’s padding bits—and what a shame that is when used with atomic compare-and-exchange.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#story"><span class="secno">1</span> <span class="content">Story Time</span></a>
     <ol class="toc">
      <li><a href="#struct"><span class="secno">1.1</span> <span class="content">The Curious Case of <code class="highlight"><c- k>struct</c-></code></span></a>
      <li><a href="#union"><span class="secno">1.2</span> <span class="content">A <code class="highlight"><c- k>union</c-></code> Born in Unusual Circumstances</span></a>
      <li><a href="#fp"><span class="secno">1.3</span> <span class="content">Points are Floating, Even <code class="highlight"><c- n>NaN</c-></code>wards</span></a>
      <li><a href="#store"><span class="secno">1.4</span> <span class="content">You Never Know What’s Storing</span></a>
      <li><a href="#miss"><span class="secno">1.5</span> <span class="content">Defining the Opportunities We’ll Miss</span></a>
      <li><a href="#impression"><span class="secno">1.6</span> <span class="content">Making the Greatest Impression</span></a>
      <li><a href="#life"><span class="secno">1.7</span> <span class="content">Standards Can Only Be Understood Backward, they Must Be Lived Forward</span></a>
     </ol>
    <li>
     <a href="#word"><span class="secno">2</span> <span class="content">Proposed Wording</span></a>
     <ol class="toc">
      <li><a href="#word-meta-type-synop"><span class="secno">2.1</span> <span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code> synopsis [<strong>meta.type.synop</strong>]</span></a>
      <li><a href="#word-mta-unary-prop"><span class="secno">2.2</span> <span class="content">Type properties [<strong>meta.unary.prop</strong>]</span></a>
      <li><a href="#word-atomics-types-operations-req"><span class="secno">2.3</span> <span class="content">Requirements for operations on atomic types [<strong>atomics.types.operations.req</strong>]</span></a>
      <li><a href="#word-atomics-types-generic"><span class="secno">2.4</span> <span class="content">Atomic types [<strong>atomics.types.generic</strong>]</span></a>
      <li><a href="#word-atomics.syn"><span class="secno">2.5</span> <span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>atomic</c-><c- o>></c-></code> synopsis [<strong>atomics.syn</strong>]</span></a>
     </ol>
    <li><a href="#sample"><span class="secno">3</span> <span class="content">Sample Program</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <p>This issue has been discussed by the authors at every recent Standards meetings,
yet a full solution has been elusive despite helpful proposals. We believe that
this proposal can fix this oft-encountered problem once and for all.</p>
   <h2 class="heading settled" data-level="1" id="story"><span class="secno">1. </span><span class="content">Story Time</span><a class="self-link" href="#story"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="struct"><span class="secno">1.1. </span><span class="content">The Curious Case of <code class="highlight"><c- k>struct</c-></code></span><a class="self-link" href="#struct"></a></h3>
   <p>Using the Standard’s atomic compare-and-exchange on <code class="highlight"><c- k>struct</c-></code>s with padding bits
is fraught with peril: the <code class="highlight"><c- n>desired</c-></code> value is passed by value to all of the
methods, which doesn’t guarantee than any particular bit pattern is preserved in
the bits which don’t participate in the <em>value representation</em>. the
padding bits mean that there are multiple <em>object representations</em> for
the same value representation.</p>
   <p>Indeed:</p>
   <ul>
    <li data-md>
     <p>Object initialization doesn’t guarantee any particular value for padding bits.</p>
    <li data-md>
     <p>Copying an object can change its padding bits, on <strong>every copy</strong>.</p>
   </ul>
   <p>Whereas using compare-and-exchange acts on the <strong>entire</strong> object,
padding bits included. From 29.6.5 Requirements for operations on atomic types
[<strong>atomics.types.operations.req</strong>], ¶24:</p>
   <blockquote>
     [ <em>Note:</em> For example, the effect of <code class="highlight"><c- n>atomic_compare_exchange_strong</c-></code> is 
<pre class="highlight"><c- k>if</c-> <c- p>(</c-><c- n>memcmp</c-><c- p>(</c-><c- n>object</c-><c- p>,</c-> <c- n>expected</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- o>*</c-><c- n>object</c-><c- p>))</c-> <c- o>==</c-> <c- mi>0</c-><c- p>)</c->
    <c- n>memcpy</c-><c- p>(</c-><c- n>object</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>desired</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- o>*</c-><c- n>object</c-><c- p>));</c->
<c- k>else</c->
    <c- nf>memcpy</c-><c- p>(</c-><c- n>expected</c-><c- p>,</c-> <c- n>object</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- o>*</c-><c- n>object</c-><c- p>));</c->
</pre>
    <p><em>— end note</em> ]</p>
   </blockquote>
   <p>Compare-and-exchange is specified this way because that’s how hardware works: <code class="highlight"><c- n>cmpxchg</c-></code> or load-linked / store-conditional instuctions must operate on the
entire address range to be atomic.</p>
   <p>The interactions between non-atomic and atomic objects which contain padding
bits means that compare-and-exchange is <strong>never</strong> guaranteed to
succeed, even if the program only ever uses a single value representation.</p>
   <p>We’ve included a <a href="#sample">§ 3 Sample Program</a> which exhibits this behavior.</p>
   <h3 class="heading settled" data-level="1.2" id="union"><span class="secno">1.2. </span><span class="content">A <code class="highlight"><c- k>union</c-></code> Born in Unusual Circumstances</span><a class="self-link" href="#union"></a></h3>
   <p>Unions can exhibit a similar problem: they can contain padding, but can also
contain bits which don’t participate in their active member’s value
representation.</p>
   <h3 class="heading settled" data-level="1.3" id="fp"><span class="secno">1.3. </span><span class="content">Points are Floating, Even <code class="highlight"><c- n>NaN</c-></code>wards</span><a class="self-link" href="#fp"></a></h3>
   <p>Astute readers will note that value representations with multiple object
representations exist for more than simply <code class="highlight"><c- k>struct</c-></code> and <code class="highlight"><c- k>union</c-></code>. Even with <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code>'s requirement that objects be trivially copyable, it is possible
for boolean, signed integral and floating-point values to suffer from this
problem.</p>
   <p>The common example where this happens is when signaling <code class="highlight"><c- n>NaN</c-></code>s are present: some
platforms canonicalize them to quiet a <code class="highlight"><c- n>NaN</c-></code>. This is a similar problem to
padding bits: copying can change the object representation.</p>
   <h3 class="heading settled" data-level="1.4" id="store"><span class="secno">1.4. </span><span class="content">You Never Know What’s Storing</span><a class="self-link" href="#store"></a></h3>
   <p>Another surprising case is when atomic <code class="highlight"><c- n>store</c-></code> or <code class="highlight"><c- n>exchange</c-></code> operations are
implemented using compare-and-exchange. This happens when the target
architecture doesn’t have a suitably-sized <code class="highlight"><c- n>store</c-></code> or <code class="highlight"><c- n>exchange</c-></code> instructions,
but has an appropriate <code class="highlight"><c- n>cmpxchg</c-></code> or load-linked / store-conditional. To
guarantee lock-freedom, implementations rely on compare-and-exchange. This is
transparent to the developer, but can lead to permanent failure to <code class="highlight"><c- n>store</c-></code> or <code class="highlight"><c- n>exchange</c-></code> if padding bits are present.</p>
   <p>The same implementation approach is possible for <code class="highlight"><c- n>load</c-></code>s, but failure to perform
an idempotent compare-and-exchange is the same as a <code class="highlight"><c- n>load</c-></code>, regardless of
padding bits since it yields the previous value.</p>
   <h3 class="heading settled" data-level="1.5" id="miss"><span class="secno">1.5. </span><span class="content">Defining the Opportunities We’ll Miss</span><a class="self-link" href="#miss"></a></h3>
   <p>This problem is valid for <code class="highlight"><c- k>union</c-></code>, some scalar representations, and some <code class="highlight"><c- n>store</c-></code> / <code class="highlight"><c- n>exchange</c-></code> implementations. The authors nonetheless believe that it is a
separable from that of padding bits on <code class="highlight"><c- k>struct</c-></code>s. Indeed, all <code class="highlight"><c- k>struct</c-></code> with
padding bits exhibit this problem, whereas only limited scalar values are
affected. The same applies for <code class="highlight"><c- k>union</c-></code>: the issue can only surface when the
widest member isn’t the active member (though <code class="highlight"><c- k>union</c-></code>s do suffer of the padding
problem). Only some <code class="highlight"><c- n>store</c-></code> / <code class="highlight"><c- n>exchange</c-></code> implementations are affected, in which
case their <code class="highlight"><c- k>struct</c-></code>s with padding bits are also affected. We don’t believe <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- b>bool</c-><c- o>></c-></code> suffers from this problem.</p>
   <p>Put another way: compare-and-exchange of a <code class="highlight"><c- k>struct</c-></code> with padding bits is <strong>always</strong> wrong, whereas the other similar issues are <strong>sometimes</strong> wrong.</p>
   <p>This leads us to the following conclusions:</p>
   <ol>
    <li data-md>
     <p>For types with padding bits a simple solution exists using the existing
 Standard: developers should explicitly define padding members if they
 intend to compare-and-exchange a type. Were an approach such as <a data-link-type="biblio" href="#biblio-n3986">[N3986]</a> adopted, developers could rely on this facility to avoid padding bits more
 easily.</p>
    <li data-md>
     <p>Other types which don’t have unique object representations merit another
 solution: developers should never compare-and-exchange a value which has
 multiple object representations. All other values of that type can be
 compare-and-exchanged without issue.</p>
    <li data-md>
     <p>Implementations which rely on compare-and-exchange for the <code class="highlight"><c- n>store</c-></code> and <code class="highlight"><c- n>exchange</c-></code> functions of certain instantiations of <code class="highlight"><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> could apply a
 similar solution to that proposed in this paper. The authors are interested
 in writing a separate paper to address the issue.</p>
   </ol>
   <p>This paper only addresses the first conclusion.</p>
   <h3 class="heading settled" data-level="1.6" id="impression"><span class="secno">1.6. </span><span class="content">Making the Greatest Impression</span><a class="self-link" href="#impression"></a></h3>
   <p>Conclusion 1. above is addressable by developers, but the authors believe that
this Standard should prevent this dangerous usage of compare-and-exchange. This
is better handled as a requirement of the methods than as a QoI warning.</p>
   <p>The authors therefore propose using concepts' <code class="highlight"><c- n>requires</c-></code> clause to all the
compare-and-exchange functions, forbidding their usage on a type which has
padding bits. All other <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code> operations would remain valid, only
compare-and-exchange would become a compilation error if used.</p>
   <p>The Standard, if <a data-link-type="biblio" href="#biblio-p0020r3">[P0020r3]</a> is adopted, will specify 4 specializations for <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-></code>:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>*></c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>integral</c-><c- o>></c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>floating</c-><c- o>-</c-><c- n>point</c-><c- o>></c-></code></p>
   </ul>
   <p>Since <a data-link-type="biblio" href="#biblio-p0258r2">[P0258r2]</a> the Standard contains the type trait <code class="highlight"><c- n>has_unique_object_representations</c-></code>, as defined in 20.15.4.3 Type properties
[<strong>meta.unary.prop</strong>] ¶9. This trait initially seems ideally suited
to our purpose:</p>
   <blockquote>
    <p>The predicate condition for a template specialization <code class="highlight"><c- n>has_unique_object_representations</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-></code> shall be satisfied if and only
  if:</p>
    <ul>
     <li data-md>
      <p>T is trivially copyable, and</p>
     <li data-md>
      <p>any two objects of type <code class="highlight"><c- n>T</c-></code> with the same value have the same object
representation, where two objects of array or non-union class type are
considered to have the same value if their respective sequences of direct
subobjects have the same values, and two objects of union type are
considered to have the same value if they have the same active member and
the corresponding members have the same value.</p>
    </ul>
    <p>The set of scalar types for which this condition holds is
  implementation-defined. [ <em>Note:</em> If a type has padding bits, the
  condition does not hold; otherwise, the condition holds true for unsigned
  integral types. <em>— end note</em> ]</p>
   </blockquote>
   <p>We could apply it to the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> variants of compare-and-exchange only,
avoiding the pointer, integral and floating-point specializations. This
unfortunately leaves out what seems like a useful case:</p>
<pre class="highlight"><c- k>struct</c-> <c- n>T</c-> <c- p>{</c->
  <c- b>float</c-> <c- n>a</c-><c- p>;</c->
  <c- b>float</c-> <c- n>b</c-><c- p>;</c->
<c- p>};</c->
<c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-> <c- n>t</c-><c- p>;</c->
<c- k>static_assert</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>has_unique_object_representations_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-><c- p>);</c->
</pre>
   <p>This type typically has no padding bits, fits the <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> specialization, yet on some platforms does <strong>not</strong> have a unique
object representation because of its floating-point members. The above type is
often spelled as <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>complex</c-><c- o>&lt;</c-><c- b>float</c-><c- o>>></c-></code>, which is a very useful
type, even atomically. Naïvely adding <code class="highlight"><c- n>requires</c-> <c- n>has_unique_object_representations_v</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> on the compare-and-exchange members of <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code> would—among other things—forbid using compare-and-exchange on
complex numbers. The authors believe this is unacceptable.</p>
   <h3 class="heading settled" data-level="1.7" id="life"><span class="secno">1.7. </span><span class="content">Standards Can Only Be Understood Backward, they Must Be Lived Forward</span><a class="self-link" href="#life"></a></h3>
   <p>We find ourselves unable to use <code class="highlight"><c- n>has_unique_object_representations</c-></code>, which was
intended for future <code class="highlight"><c- n>std</c-><c- o>::</c-><c- n>hash</c-></code> proposals but was hoped to also be usable for
our present usecase. We thus propose a new type trait, tentatively named <code class="highlight"><c- n>has_padding_bits</c-></code>.</p>
   <p>If this sounds familar, that’s because it is:</p>
   <ul>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-p0258r2">[P0258r2]</a> introduced <code class="highlight"><c- n>has_unique_object_representations</c-></code>.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-p0258r1">[p0258r1]</a> proposed <code class="highlight"><c- n>is_contiguous_layout</c-></code>.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-p0029r0">[P0029r0]</a> proposed <code class="highlight"><c- n>is_uniquely_represented</c-></code>.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-n3980">[N3980]</a> proposed <code class="highlight"><c- n>is_contiguously_hashable</c-></code>.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-n3333">[N3333]</a> proposed <code class="highlight"><c- n>is_contiguous_layout</c-></code>.</p>
    <li data-md>
     <p><a data-link-type="biblio" href="#biblio-n4130">[N4130]</a> discussed the padding bits issue, and wondered whether a <code class="highlight"><c- n>has_padding_bits</c-></code> trait was sensible. It was reviewed by SG1 in the 2014
Redmond meeting. Guidance was sought to also resolve <a data-link-type="biblio" href="#biblio-lwg2334">[LWG2334]</a> as well as
WG14 DR 431 while keeping C compatibility, but no conclusion was reached.</p>
   </ul>
   <p>The reader now also understands this paper’s Benjamin Button theme.</p>
   <h2 class="heading settled" data-level="2" id="word"><span class="secno">2. </span><span class="content">Proposed Wording</span><a class="self-link" href="#word"></a></h2>
   <h3 class="heading settled" data-level="2.1" id="word-meta-type-synop"><span class="secno">2.1. </span><span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>type_traits</c-><c- o>></c-></code> synopsis [<strong>meta.type.synop</strong>]</span><a class="self-link" href="#word-meta-type-synop"></a></h3>
   <p>Add a new typetrait:</p>
<pre class="highlight"><ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>has_padding_bits</c-><c- p>;</c-></ins></pre>
   <p>and:</p>
<pre class="highlight"><ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>constexpr</c-> <c- b>bool</c-> <c- n>has_padding_bits_v</c-> <c- o>=</c-> <c- n>has_padding_bits</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-><c- p>;</c-></ins></pre>
   <h3 class="heading settled" data-level="2.2" id="word-mta-unary-prop"><span class="secno">2.2. </span><span class="content">Type properties [<strong>meta.unary.prop</strong>]</span><a class="self-link" href="#word-mta-unary-prop"></a></h3>
   <p>Add to Table — Type property predicates:</p>
   <table>
    <thead>
     <tr>
      <th>Template
      <th>Condition
      <th>Preconditions
    <tbody>
     <tr>
      <td>᠁
      <td>᠁
      <td>᠁
     <tr>
      <td>
<pre class="highlight"><ins><c- k>template</c-> <c- o>&lt;</c-><c- k>class</c-> <c- nc>T</c-><c- o>></c-> <c- k>struct</c-> <c- n>has_padding_bits</c-><c- p>;</c-></ins></pre>
      <td>
       <ins> For an array type <code class="highlight"><c- n>T</c-></code>, the same result as <code class="highlight"><c- n>has_padding_bits</c-><c- o>&lt;</c-><c- n>remove_all_extents_t</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>>::</c-><c- n>value</c-></code>, otherwise <em>see below</em></ins>
      <td>
       <ins> <code class="highlight"><c- n>T</c-></code> shall be a complete type, (possibly cv-qualified) <code class="highlight"><c- b>void</c-></code>, or an
      array of unknown bound. </ins>
     <tr>
      <td>᠁
      <td>᠁
      <td>᠁
   </table>
   <p>Add the following paragraph:</p>
   <blockquote>
    <ins>The predicate condition for a template specialization <code class="highlight"><c- n>has_padding_bits</c-><c- o>&lt;</c-><c- n>T</c-><c- o>>::</c-><c- n>value</c-></code> shall be satisfied if and only if:</ins>
    <ul>
     <li data-md>
      <ins><code class="highlight"><c- n>T</c-></code> is trivially copyable, and</ins>
     <li data-md>
      <ins>there exists at least one bit in the object representations of an
object of type <code class="highlight"><c- n>T</c-></code> which never participates in the value representation. </ins>
    </ul>
   </blockquote>
   <p>Or alternatively for the 2nd bullet:</p>
   <blockquote>
    <ul>
     <li data-md>
      <ins>given two objects of type <code class="highlight"><c- n>T</c-></code>, for each bit in their object
representations that bit does not participate in their value
representations.</ins>
    </ul>
   </blockquote>
   <h3 class="heading settled" data-level="2.3" id="word-atomics-types-operations-req"><span class="secno">2.3. </span><span class="content">Requirements for operations on atomic types [<strong>atomics.types.operations.req</strong>]</span><a class="self-link" href="#word-atomics-types-operations-req"></a></h3>
   <p>Precede each of the following functions:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_weak</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_strong</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_weak_explicit</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_strong_explicit</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>A</c-><c- o>::</c-><c- n>atomic_compare_exchange_weak</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>A</c-><c- o>::</c-><c- n>atomic_compare_exchange_strong</c-></code></p>
   </ul>
   <p>
    With this concept: 
    <ins>requires <code class="highlight"><c- n>has_padding_bits_v</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code></ins>
    .
   </p>
   <p>Update the following paragraph:</p>
   <blockquote>
    <p>
     <em>Requires:</em> The <code class="highlight"><c- n>failure</c-></code> argument shall not be <code class="highlight"><c- n>memory_order_release</c-></code> nor <code class="highlight"><c- n>memory_order_acq_rel</c-></code>. 
     <ins><code class="highlight"><c- n>has_padding_bits_v</c-><c- o>&lt;</c-><c- n>C</c-><c- o>></c-></code> shall be <code class="highlight">false</code>.</ins>
    </p>
   </blockquote>
   <p>Update the following note:</p>
   <blockquote>
    <p>[ <em>Note:</em></p>
    <p>
     The <code class="highlight"><c- n>memcpy</c-></code> and <code class="highlight"><c- n>memcmp</c-></code> semantics of the compare-and-exchange operations may
  result in failed comparisons for values that compare equal with <code class="highlight"><c- k>operator</c-><c- o>==</c-></code> if the underlying type has 
     <del>padding bits</del>
     <ins>bits which don’t
  participate in active member’s value representation</ins>
     , trap bits, or
  alternate representations of the same value. Thus,
    </p>
    <del><code class="highlight"><c- n>compare_exchange_strong</c-></code></del>
    <ins>compare-and-exchange operations</ins>
     should be used with extreme care.
    <del> On the other hand, <code class="highlight"><c- n>compare_exchange_weak</c-></code> should converge rapidly.</del>
    <p><em>— end note</em> ]</p>
   </blockquote>
   <h3 class="heading settled" data-level="2.4" id="word-atomics-types-generic"><span class="secno">2.4. </span><span class="content">Atomic types [<strong>atomics.types.generic</strong>]</span><a class="self-link" href="#word-atomics-types-generic"></a></h3>
   <p>For all functions modified in <a href="#word-atomics-types-operations-req">§ 2.3 Requirements for operations on atomic types [atomics.types.operations.req]</a>, add the
same concepts to <code class="highlight"><c- n>atomic</c-><c- o>&lt;</c-><c- n>T</c-><c- o>></c-></code>'s <code class="highlight"><c- n>compare_exchange_weak</c-></code> and <code class="highlight"><c- n>compare_exchange_strong</c-></code> and all its specializations.</p>
   <h3 class="heading settled" data-level="2.5" id="word-atomics.syn"><span class="secno">2.5. </span><span class="content">Header <code class="highlight"><c- o>&lt;</c-><c- n>atomic</c-><c- o>></c-></code> synopsis [<strong>atomics.syn</strong>]</span><a class="self-link" href="#word-atomics.syn"></a></h3>
   <p>Add the same concept for all overloads of the following free function:</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_weak</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_strong</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_weak_explicit</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- n>atomic_compare_exchange_strong_explicit</c-></code></p>
   </ul>
   <h2 class="heading settled" data-level="3" id="sample"><span class="secno">3. </span><span class="content">Sample Program</span><a class="self-link" href="#sample"></a></h2>
   <p>This program uses compare-and-exchange on a <code class="highlight"><c- k>struct</c-></code> which has padding bits. It
may loop infinitely, or not.</p>
<pre class="highlight"><c- cp>#include</c-> &lt;atomic>
<c- cp>#include</c-> &lt;cstring>
<c- cp>#include</c-> &lt;new>
<c- cp>#include</c-> &lt;stdio.h>
<c- cp>#include</c-> &lt;type_traits>

<c- k>struct</c-> <c- n>Padded</c-> <c- p>{</c->
  <c- b>char</c-> <c- n>c</c-> <c- o>=</c-> <c- mh>0xFF</c-><c- p>;</c->
  <c- c1>// Padding here.</c->
  <c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mh>0xFEEDFACE</c-><c- p>;</c->
  <c- n>Padded</c-><c- p>()</c-> <c- o>=</c-> <c- k>default</c-><c- p>;</c->
<c- p>};</c->
<c- k>typedef</c-> <c- n>std</c-><c- o>::</c-><c- n>atomic</c-><c- o>&lt;</c-><c- n>Padded</c-><c- o>></c-> <c- n>Atomic</c-><c- p>;</c->
<c- k>typedef</c-> <c- n>std</c-><c- o>::</c-><c- n>aligned_storage</c-><c- o>&lt;</c-><c- k>sizeof</c-><c- p>(</c-><c- n>Atomic</c-><c- p>)</c-><c- o>>::</c-><c- n>type</c-> <c- n>Storage</c-><c- p>;</c->

<c- b>void</c-> <c- nf>peek</c-><c- p>(</c-><c- k>const</c-> <c- b>char</c-><c- o>*</c-> <c- n>what</c-><c- p>,</c-> <c- b>void</c-> <c- o>*</c-><c- n>into</c-><c- p>)</c-> <c- p>{</c->
  <c- n>printf</c-><c- p>(</c-><c- s>"%16s %08x %08x</c-><c- se>\n</c-><c- s>"</c-><c- p>,</c-> <c- n>what</c-><c- p>,</c-> <c- o>*</c-><c- p>(</c-><c- b>int</c-><c- o>*</c-><c- p>)</c-><c- n>into</c-><c- p>,</c-> <c- o>*</c-><c- p>(</c-><c- mi>1</c-> <c- o>+</c-> <c- p>(</c-><c- b>int</c-><c- o>*</c-><c- p>)</c-><c- n>into</c-><c- p>));</c->
<c- p>}</c->

<c- n>Storage</c-><c- o>*</c-> <c- nf>create</c-><c- p>()</c-> <c- p>{</c->
  <c- k>auto</c-><c- o>*</c-> <c- n>storage</c-> <c- o>=</c-> <c- k>new</c-> <c- n>Storage</c-><c- p>();</c->
  <c- n>std</c-><c- o>::</c-><c- n>memset</c-><c- p>(</c-><c- n>storage</c-><c- p>,</c-> <c- mh>0xBA</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>(</c-><c- n>Storage</c-><c- p>));</c->
  <c- k>asm</c-> <c- k>volatile</c-><c- p>(</c-><c- s>""</c-><c- o>:::</c-><c- s>"memory"</c-><c- p>);</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"storage"</c-><c- p>,</c-> <c- n>storage</c-><c- p>);</c->
  <c- k>return</c-> <c- n>storage</c-><c- p>;</c->
<c- p>}</c->

<c- n>Atomic</c-><c- o>*</c-> <c- nf>change</c-><c- p>(</c-><c- n>Storage</c-><c- o>*</c-> <c- n>storage</c-><c- p>)</c-> <c- p>{</c->
  <c- c1>// As if we used an allocator which reuses memory.</c->
  <c- k>auto</c-><c- o>*</c-> <c- n>atomic</c-> <c- o>=</c-> <c- k>new</c-><c- p>(</c-><c- n>storage</c-><c- p>)</c-> <c- n>Atomic</c-><c- p>;</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"atomic placed"</c-><c- p>,</c-> <c- n>atomic</c-><c- p>);</c->
  <c- n>std</c-><c- o>::</c-><c- n>atomic_init</c-><c- p>(</c-><c- n>atomic</c-><c- p>,</c-> <c- n>Padded</c-><c- p>());</c-> <c- c1>// Which bits go in?</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"atomic init"</c-><c- p>,</c-> <c- n>atomic</c-><c- p>);</c->
  <c- k>return</c-> <c- n>atomic</c-><c- p>;</c->
<c- p>}</c->

<c- n>Padded</c-> <c- nf>infloop_maybe</c-><c- p>(</c-><c- n>Atomic</c-><c- o>*</c-> <c- n>atomic</c-><c- p>)</c-> <c- p>{</c->
  <c- n>Padded</c-> <c- n>desired</c-><c- p>;</c->  <c- c1>// Padding unknown.</c->
  <c- n>Padded</c-> <c- n>expected</c-><c- p>;</c-> <c- c1>// Could be different.</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"desired before"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>desired</c-><c- p>);</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"expected before"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>expected</c-><c- p>);</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"atomic before"</c-><c- p>,</c-> <c- n>atomic</c-><c- p>);</c->
  <c- k>while</c-> <c- p>(</c->
    <c- o>!</c-><c- n>atomic</c-><c- o>-></c-><c- n>compare_exchange_strong</c-><c- p>(</c->
      <c- n>expected</c-><c- p>,</c->
      <c- n>desired</c-> <c- c1>// Padding bits added and removed here ˙ ͜ʟ˙</c->
  <c- p>));</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"expected after"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>expected</c-><c- p>);</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"atomic after"</c-><c- p>,</c-> <c- n>atomic</c-><c- p>);</c->
  <c- k>return</c-> <c- n>expected</c-><c- p>;</c-> <c- c1>// Maybe changed here as well.</c->
<c- p>}</c->

<c- b>int</c-> <c- nf>main</c-><c- p>()</c-> <c- p>{</c->
  <c- k>auto</c-><c- o>*</c-> <c- n>storage</c-> <c- o>=</c-> <c- n>create</c-><c- p>();</c->
  <c- k>auto</c-><c- o>*</c-> <c- n>atomic</c-> <c- o>=</c-> <c- n>change</c-><c- p>(</c-><c- n>storage</c-><c- p>);</c->
  <c- n>Padded</c-> <c- n>p</c-> <c- o>=</c-> <c- n>infloop_maybe</c-><c- p>(</c-><c- n>atomic</c-><c- p>);</c->
  <c- n>peek</c-><c- p>(</c-><c- s>"main"</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>p</c-><c- p>);</c->
  <c- k>return</c-> <c- mi>0</c-><c- p>;</c->
<c- p>}</c->
</pre>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-lwg2334">[LWG2334]
   <dd>Daniel Krügler. <a href="https://wg21.link/lwg2334">atomic's default constructor requires "uninitialized" state even for types with non-trivial default-constructor</a>. SG1. URL: <a href="https://wg21.link/lwg2334">https://wg21.link/lwg2334</a>
   <dt id="biblio-n3333">[N3333]
   <dd>J. Yasskin, C. Carruth. <a href="https://wg21.link/n3333">Hashing User-Defined Types in C++1y</a>. 13 January 2012. URL: <a href="https://wg21.link/n3333">https://wg21.link/n3333</a>
   <dt id="biblio-n3980">[N3980]
   <dd>H. Hinnant, V. Falco, J. Byteway. <a href="https://wg21.link/n3980">Types don't know #</a>. 24 May 2014. URL: <a href="https://wg21.link/n3980">https://wg21.link/n3980</a>
   <dt id="biblio-n3986">[N3986]
   <dd>S. Davalle, D. Gutson, A. Bustamante. <a href="https://wg21.link/n3986">Adding Standard support to avoid padding within structures</a>. 25 April 2014. URL: <a href="https://wg21.link/n3986">https://wg21.link/n3986</a>
   <dt id="biblio-n4130">[N4130]
   <dd>JF Bastien, O. Giroux. <a href="https://wg21.link/n4130">Pad Thy Atomics</a>. 1 September 2014. URL: <a href="https://wg21.link/n4130">https://wg21.link/n4130</a>
   <dt id="biblio-p0020r3">[P0020r3]
   <dd>H. Carter Edwards, Hans Boehm, Olivier Giroux, JF Bastien, James Reus. <a href="https://wg21.link/p0020r3">Floating Point Atomic View</a>. 14 October 2016. URL: <a href="https://wg21.link/p0020r3">https://wg21.link/p0020r3</a>
   <dt id="biblio-p0029r0">[P0029r0]
   <dd>Geoff Romer, Chandler Carruth. <a href="https://wg21.link/p0029r0">A Unified Proposal for Composable Hashing</a>. 21 September 2015. URL: <a href="https://wg21.link/p0029r0">https://wg21.link/p0029r0</a>
   <dt id="biblio-p0258r1">[P0258R1]
   <dd>Michael Spencer. <a href="https://wg21.link/p0258r1">is_contiguous_layout</a>. 5 March 2016. URL: <a href="https://wg21.link/p0258r1">https://wg21.link/p0258r1</a>
   <dt id="biblio-p0258r2">[P0258r2]
   <dd>Michael Spencer. <a href="https://wg21.link/p0258r2">has_unique_object_representations - wording</a>. 24 June 2016. URL: <a href="https://wg21.link/p0258r2">https://wg21.link/p0258r2</a>
  </dl>